The present invention is related to a device and method for starting a low supply bandgap reference circuit.
The objective of a bandgap reference circuit is to provide a voltage that remains constant when the temperature changes. The bandgap reference circuit generates a stable voltage over a temperature range by utilizing two semiconductor circuits, one for providing a voltage that is proportional to absolute temperature (PTAT) and a second for providing a voltage that is complementary to absolute temperature (CTAT). Conventionally the sum of the two circuits is used to provide a temperature-stabilized voltage reference.
FIG. 1 shows a simplified circuit of a conventional bandgap reference circuit 100. Node A provides a voltage, which is complementary to absolute temperature (CTAT) based on the negative temperature dependent junction voltage of a PN diode, which is about −1.5 mV/° C. Node B provides a large area PN-type device 104 in series with resistor 102 to ground. The feedback loop comprising an opamp 106 and a pair of matched controlled current sources P1 and P2 forces the voltages at node A and node B to be equal. According to the I-V equation of a PN diode, the voltages at nodes A and B, Va and Vb are:Ia=Aa*I0*exp(qVa/kT)Ib=N*Aa*I0*exp(q(Vb−Ib*Rb)/kT)If Va is set to be equal to Vb, and Ia=Ib, the above two equations can be simplified asIb*Rb=kT/q*ln(N),so that the current Ib flowing through node B is proportional to absolute temperature.
The output voltage Vbg developed across a resistor in the output stage 110, is a PTAT current Ic, mirrored from Ib, on Rc in series with a negative-temperature-coefficient diode voltage. The Vbg could be designed to be temperature independent if the magnitudes of Ic and Rc are proper to compensate the negative temperature coefficient of a diode.
To reduce power consumption, the feedback loop is generally self-biased. Like other self-biasing circuits, the bandgap reference circuit 100 may have two stable states. The first stable state is when it begins normal operation as designed, and the second stable state is when all the currents are zero (or floating). The circuit can be at zero current when the bandgap circuit initially powers up or as a result of power interruptions. When this zero current state occurs, the bandgap circuit is in a non-started state and the bandgap voltage (Vbg) is improper. A “startup” circuit may be employed to ensure the bandgap reference circuit starts. The purpose of a startup circuit is to ensure the proper operational state can be set during power up without interfering with normal operation of the bandgap circuit once it is started.
FIG. 2 shows a simplified schematic of a conventional bandgap reference circuit 200 with a startup circuit. The bandgap reference circuit 200 is comprised of a diode 108 for providing a CTAT voltage at the node A, a plurality of diodes 104 in series with resistor 102 for providing a PTAT voltage at the node B, and an opamp 106 for controlling two PMOS devices P1 and P2 and providing proper biasing to the PMOS device P3 in the output stage. In the figure the opamp is shown as discrete devices, however, other opamp circuitry may be used. The PMOS devices P1 and P2 provide currents to the nodes A and B. The startup circuit comprises an NMOS device N1 coupled between a complementary power supply such as a ground or VSS, and the gates of the PMOS devices P1 and P2. The NMOS device N1 is controlled by power on a reset signal PONRST generated externally to the circuit.
To operate the power on reset of the bandgap reference circuit 200, the signal PONRST is controlled to a logical “high” such that the NMOS device N1 is turned on. Turning on the NMOS device N1 biases the PMOS devices P1 and P2 into conduction such that they provide a current to the nodes A and B. Once the current passes through either the node A or the node B, the voltages at the nodes A and B are established and the bandgap circuit is brought out of the non-started state and begins normal operation. The drawback to the power on a reset circuit is that is depends on an external signal PONRST to start the bandgap reference circuit.
The deficiencies of the conventional circuitry and methods for starting a bandgap circuit show that a need still exists for improvement. To overcome the shortcomings of the conventional circuitry, new circuitry and method for starting a bandgap circuit is needed.